In recent years, automobiles running on electric power such as an electric vehicle (EV), a plug-in hybrid electric vehicle (PHEV), and a hybrid electric vehicle (HEV) (hereinafter simply referred to as “vehicle” or “vehicles”) have become widespread. Such vehicles are mounted with a large capacity storage battery, store electric energy supplied from outside in a storage battery and run using the stored electric energy.
As a method of supplying power to a storage battery of a vehicle from the outside, a method is known whereby power is supplied between a primary side coil of a power supply apparatus provided on a ground side and a secondary side coil of a power receiving apparatus provided on a vehicle side using an electromagnetic force in a wireless manner. According to this wireless power supply method, it is known that a frequency characteristic for efficiency of power supply (hereinafter, referred to as “power supply efficiency”) when an inter-coil coupling coefficient is high is a double hump characteristic having two peaks (resonance points) (e.g., see PTL 1). In wireless power supply for vehicles in particular, the shape of double hump characteristic varies depending on a gap, displacement and battery condition, and the peak positions may also constantly change, and so it is necessary to check these changes every time.
Patent Literature (hereinafter, referred to as “PTL”) 1 discloses a technique of sequentially changing a frequency of power to be sent from a power transmission coil, detecting a frequency characteristic of power received by a power receiving coil, and transmitting power using a frequency corresponding to maximum received power.